This invention relates to electronic circuits and, more particularly to power consumption within electronic circuits.
Integrated circuits are designed to meet speed requirements under worst-case operating conditions. In Lucent Technology's 0.35 .mu.m 3.3V CMOS technology, the "worst-case-slow" condition is specified for a temperature of 125C. and a chip supply voltage, V.sub.dd, of 2.7V. The worst-case power consumption of the chip is quoted at the maximum supply voltage of 3.6V. The difference in chip performance at the "worst-case slow", nominal, and "worst-case-fast" conditions is shown in FIG. 1, where the frequency of a 25-stage ring oscillator is shown at different supply voltages and process corners. At the nominal operating voltage of 3.3V, the speed difference between "worst case slow" (WCS) and "worst case fast" (WCF) is a factor of 2.2. From the graph it can be seen that if a chip is designed to operate at 140 MHz and at 2.1V supply even when it is "worst-case-slow", a manufactured chip whose characteristics happen to be nominal will continue to operate at 140 MHz even when the chip supply is reduced to 2.1V.
The power consumption of a CMOS circuit increases linearly with operating frequency and quadratically with supply voltage. Therefore, a reduction in supply voltage can significantly reduce power consumption. For example, by reducing the nominal operating voltage from 3.3V to 2.1 V, the nominal power consumption of a 140 MHz chip is reduced by 60% without altering the circuit. This, of course, presumes an ability to identify and measure a chip's variation from nominal characteristics, and an ability to modify the supply voltage based on this measurement.
To achieve variable power supply voltage scaling, a programmable dc--dc converter may be used. Probably, the most efficient approach in use today is the buck converter circuit. These are well known in the art.
Voltage scaling as a function of temperature has been incorporated into the Intel Pentium product family as a technique to achieve high performance at varying operating temperatures and process corners. It is described in U.S. Pat. No. 5,440,520. The approach uses an on-chip temperature sensor and associated processing circuitry which issues a code to the off-chip power supply to provide a particular supply voltage. The process variation information is hard-coded into each device as a final step of manufacturing. This approach has the disadvantage of costly testing of each chip to determine its variance from nominal processing. Several manufacturers make Pentium-compatible dc--dc converter circuits, which are highlighted in "Powering the Big Microprocessors", by B. Travis, EDN, Aug. 15, pp. 31-44, 1997.
Recently, there has been considerable interest in integrating much of the buck controller circuit onto the chip. The only off-chip components are the inductor (typically about 10 .mu.H) and capacitor (typically about 30 .mu.F) used in the buck converter. Efficiencies in excess of 80% are typical for a range of voltages and load currents. See, for example, "A High-Efficiency Variable Voltage CMOS Dynamic dc--dc Switching Regulator," by W. Namgoong, M. Yu, and T. Meng, Proceedings ISSCC97 pp. 380-381, February, 1997. Researchers have been also experimenting with on-chip voltage scaling techniques to counter process and temperature variations. See "Variable Supply-Voltage Scheme for Low Power High-Speed COMS Digital Design," by T. Kuroda et al, CICC97 Conference Proceedings, and JSSC Issue of CISS97, May, 1998. The Kuroda et al paper demonstrates that the speed of the circuit can be maintained (or at least the speed degradation can be minimized) by tuning the threshold voltages even as the supply voltage is lowered. The tuning is achieved on-chip by varying the substrate-bias voltage. These techniques are needed to ensure that the leakage current, which increasing as the threshold voltage is reduced, does not become too large.
Thus, it is known that varying supply voltage to a chip can improve performance by eliminating unexpected variability in the supply voltage, and by accounting for process and operating temperature variations.